[1] It has been suggested that drift loss to the magnetopause can be one of the major loss mechanisms contributing to relativistic electron flux dropouts. In this study, we examine details of relativistic electrons' drift physics to determine the extent to which the drift loss through the magnetopause is important to the total loss of the outer radiation belt. We have numerically computed drift paths of relativistic electrons' guiding center for various pitch angles, various measurement positions, and different solar wind conditions using the Tsyganenko T02 model. We specifically demonstrate how the drift loss effect depends on these various parameters. Most importantly, we present various estimates of relative changes of the omnidirectional flux of 1 MeV electrons between two different solar wind conditions based on a simple form of the directional flux function. For a change of the dynamic pressure from 4 nPa to 10 nPa with a fixed IMF B Z = 0 nT, our estimate indicates that after this increase in pressure, the equatorial omnidirectional flux at midnight near geosynchronous altitude decreases by $56 to $97%, depending on the specific pitch angle dependence of the directional flux. The effect rapidly decreases at regions earthward of geosynchronous orbit and shows a general trend of decrease away from midnight. For a change of the IMF B Z from 0 nT to À15 nT with a fixed dynamic pressure of 4 nPa, the relative decrease of the omnidirectional flux at geosynchronous altitude on the nightside is much smaller than that for the pressure increase, but its effect becomes substantial only beyond geosynchronous orbit. Possibilities exist that our results may change to some extent for a different magnetospheric model than the one used here.

[1] Satellite observations often show that relativistic electron fluxes that decrease during a geomagnetic storm main phase do not recover their prestorm level even when the storm has substantially recovered. A possible explanation for such sustained flux dropout is that the electrons that move to larger shells (L shells) aided by the disturbance storm time (Dst) effect associated with the main phase geomagnetic field depression may be suffering drift loss to the magnetopause, resulting in irreversible (nonadiabatic) flux decreases during a geomagnetic storm. In this study, we have numerically evaluated the drift loss effect by combining it with the Dst effect and including off-equatorially mirroring electrons for three different storm conditions obtained by averaging 95 geomagnetic storms that occurred from 1997 to 2002. Using the Tsyganenko T02 model and our own simplified method, we estimated the storm time flux changes based on the guiding center orbit dynamics. Assuming that there is no competing source mechanism taking place at the same time, our calculations of the electron fluxes at equatorial midnight suggest that the drift loss when combined with the Dst effect can be responsible for flux dropouts, which can be seen even inside the geosynchronous orbit during storm periods. Specifically, by evaluating omnidirectional flux values at three specific times that correspond to the storm onset time, the time of minimum Dst value, and the end of the Dst recovery, we have obtained the following numerical results. First, for the strong storm with −150 nT < Dst min ≤ −100 nT, the combined drift loss and Dst effect can cause a complete dropout of the electron flux for r ≥ ∼5 R E at the end of the storm recovery. A nearly full recovery of the particle flux by the adiabatic Dst effect is seen only for r < ∼5 R E . For the moderate storm with −100 nT < Dst min ≤ −50 nT, the overall flux decrease level at the end of the storm recovery is less significant compared to that of the strong storm. However, the combined loss effect can still penetrate into r ∼ 5 R E , leading to some partial dropout of the flux. For the severe storm with Dst min ≤ −150 nT, the flux dropout is far more significant than for the other two storms, indicating that the combined drift loss and Dst effect can even reduce the flux level at an inner region of r ∼ 4 R E . But in this case, the solar wind dynamic pressure is so high that the dayside magnetopause can cross the geosynchronous orbit. Consequently, the flux dropouts seen in actual observations can be primarily attributed to a fast and direct loss to the magnetopause at times when the magnetopause crosses the geosynchronous orbit. It is possible that our numerical results may quantitatively change to some extent with different magnetospheric models and assumptions and may also change depending on the validity of the fully adiabatic invariants assumption.Citation: Kim, K. C., D.-Y. Lee, H.-J. Kim, E. S. Lee, and C. R. Choi (2010), Numerical estimates of drift loss and Dst effect for outer rad...

[1] It has been known that (untrapped) ring current particles can be lost through the dayside magnetopause into the magnetosheath, which is regarded as one of the major mechanisms contributing to the ring current decay. In this paper, we suggest that the solar wind dynamic pressure can play a significant role in the dayside loss in a new aspect. In order to show that, we have first analyzed the average characteristics of the dynamic pressure based on 95 geomagnetic storm events selected from the period 1997-2002. We find that the dynamic pressure overall enhances during the magnetic storm. The enhancement is most significant during the storm main phase compared to the prestorm and recovery phases, and it is higher for stronger storms. Using one of the most recent Tsyganenko models, T01s, we show that this enhanced dynamic pressure that pushes the magnetopause to move inward leads to a reduction of the scale length of the gradient of the magnetic field magnitude along the magnetopause. This results in the enhancement of the magnetic drift speed across the magnetopause. On the basis of the test particle orbit calculation, we explicitly show that this effect can be a significant factor for the particles to effectively cross the magnetopause. It applies to the adiabatic particles that have a relatively ''small'' gyroradius near the magnetopause compared to the magnetopause thickness. These particles cross the magnetopause by some number of the magnetic gradient drift motion, being in contrast to the particles with a relatively ''large'' gyroradius that can enter into the magnetosheath by crossing the magnetopause with less than one gyromotion. We argue that this can often apply to a substantial population of the ring current particles.